Carbon fiber features low specific gravity, high tensile strength, high modulus, high electric conductivity, and high thermal conductivity and has the advantage of a soft fiber that can be woven. Among the carbon fibers, a special carbon fiber with a high modulus is used extensively as an enhanced composite material for construction, aviation, and military. There are various kinds of raw materials of carbon fibers, such as rayon, polyvinyl alcohol, vinylidene chloride, polyacrylonitrile (PAN) and pitch. At present, the mainstream carbon fiber adopts polyacrylonitrile (PAN) as the raw material, and such carbon fiber has excellent mechanical strength, high quality and performance and can be manufacture stably.
The manufacturing flow of a carbon fiber using PAN as the raw material is briefly described as follows. Spinning Process (PAN raw material)→Stabilization Process (200˜300° C., oxidized in air for 10˜20 hours)→Carbonization Process (1000˜1500° C., heated in nitrogen for more than 2 hours)→Graphitization Process (1500˜3000° C., heated in argon for more than 0.5 hour)→Graphitization of Fiber. Wherein, the purpose of the graphitization process is to achieve over 90% of carbon content in the fiber and form a two-dimensional carbon ring planar mesh structure and a graphite layer structure with parallel layers. In general, an X-ray diffraction (XRD) and a Raman spectrum are used for learning the microscopic structure of the PAN carbon fibers and studying the effect of the microscopic structure of the PAN carbon fibers on the mechanical performance. In the XRD analysis, the full width at half maximum (FWHM) β of the peak value of the graphite phase is used for determining the stack height (grain thickness) of a crystal surface (002) of the graphite layer, which is generally represented by Lc as shown in the following equation (1).Lc=Kλ/β cos θ  Equation (1)
Where, K is the form factor, λ, is the wavelength of X-ray, and θ is the scattering angle.
The greater the Lc, the more stack layers is the graphite layer, and the closer is the fiber structure. Research results show that high strength carbon fibers (such as the T-series carbon fibers manufactured by Toray Company) has a crystal area composed of approximately 5 to 6 planar layers of graphite, and high strength high modulus carbon fibers (such as the MJ series carbon fibers) have a crystal area composed of approximately 10 to 20 planar layers of graphite. Theories and actual product inspection show that the greater the grin thickness of the graphite layer, the higher is the tensile modulus of the carbon fiber (as shown in Table 1).
TABLE 1Tensile Strength/Tensile Modulus/Lc/La/Model No.GPaGPaangstromangstromT3003.5323018.340.1T7004.9023020.841.3T8005.4929421.443.1T10006.3729421.945.0M404.4137736.166.7M504.054059.680.5M603.9258868.692.7
Japan Toray Company provided in a carbon fiber with tensile modulus of 180˜220 GPa and Lc of 13˜18 angstroms as disclosed in R.O.C. Pat. Application No. 94107132), indicating that Lc can be used as a standard for determining a carbon fiber structure, but the manufacturing process is the same as those disclosed process for commercial products, wherein a thermoelectric heating method is used, and the heat energy of a heat source in a furnace is radiated an/or conducted to the carbon fiber to heat the carbon fiber slowly. Carbon fiber strands are heated gradually according to different set temperatures, and pre-oxidized fibers, carbon fibers as well as graphite fibers have certain limitations.
Conventional heating and carbonization methods as disclosed in Japan Pat. No. JP200780742, R.O.C. Pat. Nos. 561207, 200902783 and 279471 focus on improving the manufacturing method that adopt a conventional thermoelectric furnace. In other words, a high temperature furnace is used for heating in the carbonization process, and different heat exchange methods are used to transmit heat energy from the outside to the inside while heating the external cavity, insulation facility, protective atmosphere and fiber. However, the drawbacks of the conventional methods include low heat conduction, difficult insulation, taking too much time to heat to the desired temperature since the temperature rising speed is affected by the heat conduction effect, and the thermal efficiency for the carbonization and graphitization process is low. Such heating method not only takes a long time, but also wastes unnecessary energy. In addition, a large quantity of insulation devices is required for a good heat insulation system to prevent heat loss of the high temperature electric furnace. The conventional methods require higher equipment requirements and costs, and thus the mass production is more difficult, and the cost of carbon fibers is higher.
Among the aforementioned prior arts, a microwave induces heating to provide a high temperature for the carbonization, and such method is generally applied in carbonization related processes as disclosed in U.S. Pat. Nos. 4,197,282, and 6,372,192 and WO Pat. No. 101084. In U.S. Pat. No. 4,197,282 issued to English Company Petroleum, a microwave carbonization process is used for processing fibers manufactured from natural organic matters such as pitch, coal, or cellulose. In the manufacturing process, a pre-carbonization process takes place at a high temperature from 300° C. to 1500° C. in an inert gas atmosphere, and then the pre-carbonized fibers are put into an inert gas and carbonized by microwave. The drawbacks of this method reside on that the pre-carbonization process conducted in the traditional high temperature furnace takes a long time (more than 4 hours) to form pre-carbonized fibers before the microwave carbonization takes place, so as to increase the level of difficulty of the manufacturing process. In addition, the precursor is a processing substance with low carbon content, so that a high strength high modulus material cannot be formed by the quick carbonization. In U.S. Pat. No. 6,372,192B1 issued to Oak Ridge Lab, a microwave plasma carbonized polyacrylonitrile (PAN) fiber is disclosed and characterized in that after the PAN fiber is pre-oxidized at 500° C., microwave is excited at low-pressure vacuum environment to produce plasma, and the plasma is used for carbonizing the pre-oxidized PAN fiber under an oxygen free environment, and the microwave energy is mainly used for producing gas plasma, and the main heating area is the surface of the fiber, and the heat capacity hardly can perform mass production of large-bundle fibers. Further, the maximum strength is only 2.3 GPa, and the modulus is only 192 GPa, and both fail to meet the high modulus specification.